1. Field of the Invention
The present invention relates to an X-ray imaging tube, and more particularly to the electrodes incorporated in the envelope of the X-ray imaging tube.
2. Description of the Related Art
An X-ray imaging tube is a device which comprises an input screen, an electrostatic electron lens system, and an output screen. The input screen has a phosphor layer and a photoelectric layer. The output screen has a phosphor layer. In operation, X-rays are applied to the input screen. The phosphor layer of the input screen converts X-rays into visible light. The photoelectric layer, which is made of alkali-antimony, converts the visible light into electrons. The electron lens system accelerates electrons and converges electron beams. The electron beams, thus converged, are applied to the phosphor layer of the output screen, which emits rays corresponding the X-rays. Hence, the X-rays applied to the input screen are observed in real time.
FIG. 1 schematically shows a high performance X-ray imaging tube in which the size of the view field can be changed. As is evident from FIG. 1, this X-ray imaging tube comprises a vacuum envelope 1. The envelope 1 comprises a metal cylinder 1a, a glass cylinder 1b, and an input window 2 made of aluminum, aluminum alloy, titanium, titanium alloy, or the like. The X-ray imaging tube further comprises an input screen 3, beam-conversing electrodes 4a, 4b and 4c, an anode 5, and an output screen 6--all located within the vacuum envelope 1. The input screen 3 faces the input window 2 and is curved along the input window 2. The anode 5 and the output screen 6 are located in the output end of the envelope 1.
The electrodes 4a, 4b and 4c are hollow cylinders for forming an electrostatic electron lens. They are coaxial with the vacuum envelope 1, spaced apart from one another in the axial direction of the envelope 1, and designed to form an X-ray image which has a uniform resolution regardless of the size of the input view field. In operation, a voltage ranging from 0 V to 25 KV is applied between the anode 5 and the photoelectric layer of the input screen 3 and the anode. In this condition, voltages are applied to the electrodes 4a, 4b and 4c, whereby these electrodes form an electron lens. The voltages applied to the electrodes 4a, 4b and 4c are changed, thus reducing the size of the view field of the X-ray imaging tube, for example, form 9 inches to 4.5 inches, from 12 inches to 6 inches, or from 14 inches 7 inches. In other words, the X-ray imaging tube shown in FIG. 1 has an image magnification of about 2.
As is shown in FIG. 2, the beam-converging electrode 4c is set at potential of about 2 KV when the magnification of used input field size is 1. This potential increases exponentially with the magnification of used input field size. As can be understood from the curve shown in FIG. 2, to increase the magnification to 2.3 or more, it is necessary to set the electrode 4c at potential of 20 KV or more. When the electrode 4c is set at 20 KV, however, the withstand voltage between the beam-converging electrodes 4b and 4c greatly decrease since the electrode 4b is set at potential of only hundreds of volts to 1.5 KV. Due to the insufficient withstand voltage, an undesirable phenomenon, such as electrical discharge or electrical leak, may occur, much impairing the ability and/or reliability of the X-ray imaging tube.
For the electrostatic electron lens system of the conventional X-ray imaging tube, it is practically impossible to provide an magnification of used input field size of 2.3 or more. To attain an magnification of used input field size of at least 2.3, at no expense of the ability or reliability, the X-ray imaging tube should be re-designed drastically.
For example, the electrode 4b can be replaced by two or more electrodes 4c.sub.1, 4c.sub.2, . . . 4c.sub.N (N .gtoreq.) as is shown in FIG. 3. In this case, these electrodes 4c.sub.1, 4c.sub.2, . . . 4c.sub.N can be set at the lowest potential, the second lowest potential, . . . and the highest potential, respectively, so that the potential difference between the beam-converging electrode 4b and the electrode 4c.sub.1 located closer to the electrode 4b than the electrodes 4c.sub.2, 2c.sub.3, . . . 4c.sub.N.
The use of more beam-converging electrodes, however, makes it more difficult to assemble the X-ray imaging tube. Moreover, the X-ray imaging tube needs to have a more complex power-supply device for applying different voltages to the beam-converging electrodes. Hence, the X-ray imaging tube cannot be manufactured at sufficiently high productivity or sufficiently low cost.